The tendency of coil springs generally to relax their resistance to compressive forces applied thereto after a period of time is well known. This is frequently due to a permanent reduction in spring length, the condition being aggravated where the spring has been insufficiently heat-treated and is employed in an elevated temperature environment. The problem becomes significant where the springs are employed as compressive, electrically conductive backstops for electrical fuses in fuse blocks, for example, where the reliability of the fuses may be directly affected. In such applications, the springs are frequently formed to terminate at the fuse contacting ends in a straight tang extending diametrically across the coils, which tang constitutes the contact surface for the fuse. At its other end, the fuse spring may terminate diametrically across the coils and is formed at a right angle in a straightened axial segment extending through the fuse socket base to form an electrical terminal. Known spring extenders of the character described, for example, in U.S. Pat. No. 2,329,591 of H. C. Christian, issued Sept. 14, 1943, designed to hook the longitudinally extending end loops of conventional coil springs are not readily adaptable to adjust such fuse block springs. An additional consideration is the fact that a fuse block spring is normally installed within a relatively inaccessible fuse socket. It is thus the problems presented by the particular character and installation of fuse block springs to which the extender tool of the present invention is chiefly directed.